1. Field of the Invention
The present invention relates to short pulse radar systems and more particularly to the minimization of susceptibility of such systems to CW narrow band jamming.
2. Description of the Prior Art
Radar systems using short pulses in the order of a nanosecond have a capability of providing range resolution and accuracy to fractions of a foot. These devices, however, though insensitive to multiple target deception and impulse type jammers, because of receiver range gates that are only a few nanoseconds wide, are vulnerable to electronic countermeasures (ECM) such as CW narrow band or spot jammers operating in the same spectral band as the baseband pulse. Incident signals from CW jammers in this range present a multiplicity of half cycles within the range gate that appear as short pulses to the system's receiver, thus providing an opportunity for the jamming signal to exceed the system threshold with each cycle. Providing a narrow range gate to reduce the probability of false alarms with each cycle of CW signal concomitantly requires a moving range gate to cover the desired over-all range, thus adding an undesired complexity. To utilize the inherent range resolution and accuracy of short pulse radar systems, a means for minimizing its susceptibility to CW narrow band jamming is required.
A prior art solution to this problem is disclosed in U.S. Pat. Application Serial No. 845,984 filed by Gerald F. Ross on Oct. 27, 1977 and assigned to the assignee of the present invention. Ross discloses a passive filter comprising a shorted stub of length l and characteristic impedance R.sub.0 /2 branching from a line of characteristic impedance R.sub.0. This configuration exhibits a large insertion loss to a CW signal with a wavelength .lambda..sub.0 when l=k .lambda..sub.0 /2, k being any integer, but passes a pulse length of .tau. with an insertion loss of 6 dB as long as 2l/v&gt;.tau., where v is the signal velocity along the line. Although this shorted stub filter provides the desired large differential insertion loss between a CW signal and a short pulse, its bandwidth is only of the order of 50 MHz. Multiple stage filter designs do not solve the problem since the differential insertion loss decreases as the bandwidth increases. Consequently, although a shorted stub may be useful in a circuit, a passive filter alone does not provide the desired immunity to spot jamming over a large dynamic range of jamming signal frequencies.
Additional improvement in anti-jam capability against narrow band CW signals may be realized by utilizing the jamming signal as a local oscillator to mix with the short pulse signal as disclosed in U.S. Pat. Application Serial No. 336,642 for "Baseband Detector with Anti-Jam Capability", filed by Cronson et al on the filing date of, and assigned to the assignee of, the instant application. Though significant anti-jam improvement is exhibited with the use of the jamming signal as a local oscillator, when two jamming signals are present within the band, however, mixing thereof may produce a signal at a difference frequency within the pass band of the system that may be of sufficient amplitude to jam the receiver. This difference frequency jamming is primarily caused by the tunnel diode bias circuitry in the receiver, which is generally of the type disclosed in U.S. Pat. No. 3,983,422, issued to Nicolson et al in Apr. 1975 and assigned to the assignee of the present invention. The adjustability of the bias circuitry in the receiver disclosed by Nicolson et al is insufficient to reduce the tunnel diode sensitivity to a level at which it does not fire in the presence of the relatively high amplitude difference frequency signal created by the mixing of the two jamming signals.